Effect of the Location of Tetanic Stimulation on Autonomic Responses: A Randomized Cross-Over Pilot Study

Background: Tetanic stimuli are used as standardized noxious inputs to investigate nociception. Previous studies have applied tetanic stimuli to various anatomical locations without validating that the resulting physiological responses were independent of the location where tetanic stimuli were applied. Our aim was to investigate the effects of three anatomical tetanic stimulus application sites on physiological variables reflecting autonomic nervous system responses as measured by photoplethysmography (PPG).Methods: Under general anesthesia, a five second, 100 hertz, 70 milliamp tetanic stimulus was applied to the ulnar nerve, medial side of the tibia, and thorax (T5 dermatome) (N=12). The effect of tetanic stimuli on PPG-derived variables (AC, DC, and ACDC) and pulse rate at each stimulus location was determined using repeated-measures analysis of variance (ANOVA) followed by Dunnett's post hoc test. Maximum tetanic stimulus-induced changes in PPG-derived variables and pulse rates were compared among the three stimulus locations using ANOVA.Results: AC and ACDC values of PPG decreased, and the DC values of PPG increased in response to tetanic stimuli-induced vasoconstriction at each location (p<0.001 for all). The maximum changes in the AC, ACDC, and DC values did not differ between locations (p=NS). There were no significant changes in pulse rate (p=NS).Conclusion: The results showed that tetanic stimulation at either of these three locations provides the same autonomic nervous system responses, as measured by PPG. Plain Language Summary: We studied how pain medications and anesthetics reduce pain in humans. The amount of pain experienced by a person can be assessed by administering a slightly uncomfortable test, such as a small electric shock to the skin. These tests are often performed before and after administering a person's pain medication or anesthesia. It is not known if the test results change when an electric shock is applied to different areas of the body (eg, hand, leg, or chest). In this study, we applied small electric shocks to three areas of skin. We measured the participants' responses to this test using a special medical equipment (pulse oximeter), which tells us how the body reacts to an uncomfortable stimulus. We found that the test results were the same regardless of the area where the electric shock was applied. This information will be useful for investigators studying pain and pain management.